Method for forming identification marks on refractory material single crystal substrate, and refractory material single crystal substrate

ABSTRACT

An identification mark formation method for forming an identification mark on a refractory material single crystal substrate that is made of one selected from the group consisting of sapphire, gallium nitride, aluminum nitride, diamond, boron nitride, zinc oxide, gallium oxide, and titanium dioxide is disclosed. The method includes: (a) scanning a principal surface of the refractory material single crystal substrate with a laser beam at a first energy density such that a groove is formed in the principal surface of the refractory material single crystal substrate, thereby forming an identification mark in the principal surface of the refractory material single crystal substrate; and (b) scanning an inside of the groove of the refractory material single crystal substrate with a laser beam at a second energy density that is lower than the first energy density.

BACKGROUND

1. Technical Field

The present invention relates to a method for forming an identificationmark on a refractory (high melting point) material single crystalsubstrate and particularly to a method for forming an identificationmark on a refractory material single crystal substrate using a laserbeam.

2. Description of the Related Art

In recent years, refractory materials, such as silicon carbide (SiC),gallium nitride (GaN), aluminum nitride (AlN), zinc oxide (ZnO), havebeen receiving attention as new materials for semiconductor devices oras new growth substrates for semiconductors. For example, the siliconcarbide semiconductor has a larger dielectric breakdown electric field,a faster saturated drift velocity of electrons, and a greater thermalconductivity than those of the silicon semiconductor. Thus, research anddevelopment have been intensively carried out for realizing a powerdevice which is capable of a large current operation at a hightemperature and at a high speed with the use of a silicon carbidesemiconductor as compared with conventional silicon devices. Amongothers, motors for use in electric motorcycles, electric vehicles, andhybrid vehicles are AC-driven or inverter-controlled, and therefore,development of efficient switching devices for such uses has beenreceiving attention. To realize such power devices, a silicon carbidesingle crystal substrate for epitaxial growth of a high-quality siliconcarbide semiconductor layer is necessary.

Demands for blue laser diodes which are used as a light source forrecording data at a high density and white diodes which are used as alight source in place of a fluorescent lamp or an incandescent bulb havebeen growing. Such light-emitting devices are manufactured using agallium nitride semiconductor, and in some cases, a wide-bandgapcompound semiconductor substrate such as a silicon carbide singlecrystal substrate is used as the substrate for formation of ahigh-quality gallium nitride semiconductor layer. Therefore, there isdemand for a wide-bandgap compound semiconductor substrate which is usedas a substrate for manufacture of a semiconductor device for whichdemand is expected to undergo a large growth in the future, such as asilicon carbide semiconductor device, a gallium nitride semiconductordevice, etc.

To a semiconductor substrate which is used for manufacture of asemiconductor device, information for identification is provided as anidentification mark for identifying semiconductor substrates andmanaging the process conditions of the manufacture process through whichthey have undergone for each of the semiconductor substrates. Usually,the identification mark has a size which is perceivable by a human eye.However, in other cases, the identification mark is imaged by a cameraor the like and image-processed so as to be detected by a semiconductormanufacturing apparatus or the like.

In forming an identification mark on a semiconductor substrate, a laserbeam is usually used. The semiconductor in a region irradiated with alaser beam is melted and evaporated, whereby a recessed portion isformed in the surface of the semiconductor substrate. The recessedportion constitutes an identification mark. According to the depth ofthis recessed portion, the method for forming an identification mark isgenerally divided into two types. Specifically, formation of anidentification mark with a recessed portion depth of about 0.1 μm to 5μm is referred to as “soft marking”, and formation of an identificationmark with a recessed portion depth of about 5 μm to 100 μm is referredto as “hard marking”. Also, in some cases, the identification mark isconstituted of a recessed portion which is in the form of an independentdot, and in other cases, the identification mark is constituted of oneor more linear grooves.

The above-described refractory semiconductor is a new semiconductormaterial and has a higher melting point than other semiconductormaterials which are widely employed, such as silicon, gallium arsenide,etc. Therefore, it is generally difficult to form a desirableidentification mark on a refractory single crystal substrate under theconditions that are suitable for formation of an identification mark ona silicon substrate. Japanese Laid-Open Patent Publication No.2006-43717 (hereinafter, referred to as Patent Document 1) discloses thetechnique of forming an identification mark which has an excellentvisibility, which is realized by irradiating a silicon carbide singlecrystal substrate with pulsed laser light which has a predeterminedpulse shape such that the silicon carbide is melted, whereby aslightly-recessed region is formed which contains a greater amount ofcarbon or silicon.

SUMMARY

According to the method of Patent Document 1, an identification markwhich is constituted of a slightly-recessed dot is formed. Therefore, itis inferred that, according to the method of Patent Document 1, theidentification mark which is constituted of the dot is formed by softmarking. Formation of the identification mark by soft marking is usuallyperformed on a mirror-finished semiconductor substrate in many cases.However, since formation of the identification mark leads to formationof a bump in the substrate, there is a problem that the flatness of thesubstrate is marred.

Patent Document 1 discloses that a recessed portion in the form of a dotwhich constitutes an identification mark is formed by a region whichcontains a greater amount of carbon or silicon, whereby theperceivability which is attributed to reflected light and transmittedlight is improved. However, there is a problem that an identificationmark which is constituted of a dot is intrinsically inferior invisibility to an identification mark which is constituted of a line.Further, the recessed portion in the form of a dot which constitutes theidentification mark has a small size, and therefore, laser dust, such asa solidified substance of silicon carbide melted by laser irradiation,abrasive grains, or other minute contaminants which can be produced inthe middle of the semiconductor manufacturing process readily remain inthe recessed portion in the form of a dot. Such contaminants remainingin the recessed portion can be a cause for contamination of the surfaceof the substrate when they are separated from the recessed portion, or acause for formation of scars in the surface, in a substratemanufacturing process or a semiconductor device manufacturing processwhich would be performed later.

The present invention solves at least one of the above problems whicharise in the prior art. One of the objects of the present invention isto provide a method for forming a highly-visible identification mark ona refractory material single crystal substrate.

An identification mark formation method of the present invention is amethod for forming an identification mark on a refractory materialsingle crystal substrate, the refractory material single crystalsubstrate being made of a single crystal which is formed by one selectedfrom the group consisting of sapphire, gallium nitride, aluminumnitride, diamond, boron nitride, zinc oxide, gallium oxide, and titaniumdioxide, the method comprising: (a) scanning a principal surface of therefractory material single crystal substrate with a laser beam at afirst energy density such that a groove is formed in the principalsurface of the refractory material single crystal substrate, therebyforming an identification mark which is constituted of one or moregrooves in the principal surface of the refractory material singlecrystal substrate; and (b) scanning an inside of the groove formed inthe principal surface of the refractory material single crystalsubstrate with a laser beam at a second energy density that is lowerthan the first energy density.

In a preferred embodiment, a width of the groove is not less than 50 μm,and a depth of the groove is not less than 10 μm.

In a preferred embodiment, the surface roughness Ra of an internalsurface of the groove is not more than 1 μm.

In a preferred embodiment, the method further comprises (c) after step(b), performing mechanical polishing on the principal surface of therefractory material single crystal substrate.

In a preferred embodiment, after step (c), gas phase etching isperformed on the principal surface of the refractory material singlecrystal substrate.

In a preferred embodiment, the surface roughness Ra of the principalsurface of the refractory material single crystal substrate is not lessthan 0.1 nm and not more than 2.0 nm.

A refractory material single crystal substrate has an identificationmark on a principal surface, the identification mark being constitutedof one or more grooves, and the refractory material single crystalsubstrate being made of a single crystal which is formed by one selectedfrom the group consisting of sapphire, gallium nitride, aluminumnitride, diamond, boron nitride, zinc oxide, gallium oxide, and titaniumdioxide, wherein a width of the groove is not less than 50 μm and lessthan 0.5 mm, and a depth of the groove is not less than 10 μm, and asurface roughness Ra of an internal surface of the groove is not morethan 1 μm.

In a preferred embodiment, a bottom surface of the groove is asolidified surface.

In a preferred embodiment, the bottom surface of the groove has astriped pattern.

According to the present invention, a refractory material single crystalsubstrate can be obtained that has an identification mark on which thereis substantially no contaminant in a groove and which has excellentvisibility.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and Figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a refractory material singlecrystal substrate in which an identification mark has been formed bymethods of the first to third embodiments for forming an identificationmark on a refractory material single crystal substrate.

FIG. 2 is a flowchart illustrating an embodiment of a method for formingan identification mark on a refractory material single crystal substrateaccording to the present invention.

FIG. 3A shows a scanning pattern of a laser beam in a rough laser step.FIG. 3B schematically shows a cross section of a groove formed in aprincipal surface of a refractory material single crystal substrate inthe rough laser step.

FIG. 4A shows a scanning pattern of a laser beam in a finishing laserstep. FIG. 4B schematically shows a cross section of a groove formed ina principal surface of a refractory material single crystal substrate inthe finishing laser step.

FIGS. 5A and 5B are a schematic plan view and a schematiccross-sectional view of a groove formed in a mechanically-polishedprincipal surface of a refractory material single crystal substrate.

FIG. 6 is a SEM image of a groove of an identification mark formed bythe method of Example 1-3.

FIGS. 7A and 7B are surface profiles of a cross section perpendicularto, and a cross section parallel to, the extending direction of a groovewhich constitutes an identification mark which was formed by the methodof Example 1-3.

FIG. 8 is a SEM image of a groove of an identification mark formed bythe method of Comparative Example 1-1.

FIGS. 9A and 9B are surface profiles of a cross section perpendicularto, and a cross section parallel to, the extending direction of a groovewhich constitutes an identification mark which is formed by the methodof Comparative Example 1-1.

FIG. 10 is an optical microscopic image of a groove of an identificationmark formed by the method of Example 2-1.

FIGS. 11A and 11B are surface profiles of a cross section perpendicularto, and a cross section parallel to, the extending direction of a groovewhich constitutes an identification mark which was formed by the methodof Example 2-1.

FIG. 12 is an optical microscopic image of a groove of an identificationmark formed by the method of Example 2-2.

FIGS. 13A and 13B are surface profiles of a cross section perpendicularto, and a cross section parallel to, the extending direction of a groovewhich constitutes an identification mark which was formed by the methodof Example 2-2.

FIG. 14 is an optical microscopic image of a groove of an identificationmark formed by the method of Comparative Example 2-1.

DETAILED DESCRIPTION

In the specification of the present application, a refractory materialsingle crystal substrate refers to a substrate (wafer) that is made of asemiconductor single crystal whose melting point is not less than 1800°C., or a single crystal substrate which is for growth of a semiconductorsingle crystal. Specifically, a refractory material single crystalsubstrate is made of a single crystal which is formed by one selectedfrom the group consisting of silicon carbide, sapphire, gallium nitride,aluminum nitride, diamond, boron nitride, zinc oxide, gallium oxide, andtitanium dioxide. The melting point, thermal conductivity, and hardness(Vickers hardness) of these materials are shown in Table 1.

TABLE 1 Sapphire SiC Al₂O₃ GaN AlN Diamond BN ZnO Ga₂O₃ TiO₂ Melting2700 2040 2500 2200 3550 2700 1975 1725-1900 1870 Point (° C.) Thermal400 42 130 340 2000 400 25 13 Conductivity (Wm⁻¹K⁻¹) Hardness 2500 23001250-1900 1700 9000 4500 400-600 750 1000 (Hv)

Each one of these refractory material single crystal substrates has arefractory. In general, this makes it difficult to form anidentification mark using a laser beam. Some of these refractorymaterial single crystal substrates whose hardness is greater than 1500Hv have bad processability. Although gallium oxide and titanium dioxidehave smaller hardnesses than 1500 Hv, gallium oxide and titanium dioxidealso have bad processability because they have high cleavability.

On the other hand, there is demand for the technique of forming anidentification mark with high visibility on such a refractory materialsingle crystal substrate, for the purpose of manufacturing newsemiconductor devices. In view of the above problems, the inventor ofthe present application conceived a novel method for forming anidentification mark on a refractory material single crystal substrateand a novel refractory material single crystal substrate which has anidentification mark.

First Embodiment

Hereinafter, the first embodiment of a method for forming anidentification mark on a refractory material single crystal substrateaccording to the present invention is described with reference to thedrawings. In the present embodiment, a silicon carbide single crystalsubstrate is used as the refractory material single crystal substrate.FIG. 1 schematically shows a silicon carbide single crystal substrate 10in which an identification mark 14 is formed by a method for forming anidentification mark on a silicon carbide single crystal substrateaccording to the first embodiment. The silicon carbide single crystalsubstrate 10 is made of silicon carbide single crystal. The polytype ofthe silicon carbide single crystal is not particularly limited. It maybe any polytype of silicon carbide single crystal. The size andthickness of the silicon carbide single crystal substrate 10 are notparticularly limited.

The silicon carbide single crystal substrate 10 has a pair of principalsurfaces 10 a and 10 b. The identification mark 14 is formed in oneprincipal surface 10 a. The plane orientation of the principal surfaces10 a and 10 b is not particularly limited. The crystal axis of thesilicon carbide single crystal and the normal lines of the principalsurfaces 10 a and 10 b may be identical with each other (so-called “justsubstrate”). Alternatively, the normal lines of the principal surfaces10 a and 10 b may form an angle which is greater than 0° with respect tothe crystal axis of the silicon carbide single crystal (so-called “offsubstrate”). The principal surface 10 a that has the identification mark14 is the rear surface, while the principal surface 10 b is the frontsurface on which a semiconductor device is to be formed.

The principal surface 10 b of the silicon carbide single crystalsubstrate 10 is preferably a mirror surface. Specifically, the surfaceroughness Ra of the principal surface 10 b is preferably not more than2.0 nm. This is because a high-quality silicon carbide layer or galliumnitride layer is epitaxially grown on the principal surface 10 b forfabrication of a semiconductor device. The lower limit of the surfaceroughness Ra of the principal surface 10 b is not particularly limited.However, as the surface roughness Ra decreases, the processing of theprincipal surface 10 b requires a longer time, so that the productivityof the silicon carbide single crystal substrate 10 deteriorates. Thus,from the viewpoint of industrial mass productivity, the surfaceroughness Ra of the principal surface 10 b is preferably not less than0.1 nm.

On the other hand, the principal surface 10 a has a surface roughnesswhich is selected according to its use or specifications required of thesilicon carbide single crystal substrate 10. Specifically, the principalsurface 10 a may be a mirror surface or may be a surface finished bymechanical polishing. When the principal surface 10 a is a mirrorsurface, the surface roughness Ra of the principal surface 10 a is notmore than 2.0 nm. When the principal surface 10 a is a surface finishedby mechanical polishing, the surface roughness Ra of the principalsurface 10 a is not less than 50 nm and not more than 1000 nm.

In the present embodiment, the identification mark 14 is formed in thevicinity of an orientation flat 12 of the silicon carbide single crystalsubstrate 10. However, the position of the identification mark 14 is notparticularly limited. The identification mark 14 may be formed at anyother position over the principal surface 10 a.

The identification mark 14 may be constituted of characters which areused in various languages, such as numerals, alphabets, Katakanacharacters, Hiragana characters, Kanji characters, etc., and symbols.The number of characters is not particularly limited. The identificationmark 14 preferably has a size which is perceivable by a naked eye. Forexample, it is preferred that the size of a single character is 0.8 mmor 1.6 mm. The upper limit of the size of a single character of theidentification mark 14 is not particularly limited. However, when thesize of a single character is excessively large, formation of characterstakes a long time. The groove width is preferably less than 0.5 mm.

As will be described in detail hereinbelow, the aforementionedalphanumeric characters which constitute the identification mark 14 arenot an identification mark which is constituted of recessed portions inthe form of dots but an identification mark which is constituted oflinear grooves. To secure sufficient visibility for a naked eye, thedepth of the groove is preferably not less than 10 μm, and the width ofthe groove is preferably not less than 50 μm. Note that the visibilitydepends on the difference in reflectance between the groove whichconstitutes the identification mark and its surrounding portion or onthe substrate on which the identification mark is formed.

Hereinafter, a method for forming an identification mark on a siliconcarbide single crystal substrate according to the present embodiment isdescribed in detail with reference to FIG. 1 and the flowchart shown inFIG. 2.

First, a silicon carbide single crystal substrate 10 is provided (stepS11). As described above, the size, thickness, and polytype of thesilicon carbide single crystal substrate 10 and the directions of thenormal lines of the principal surface 10 a and the principal surface 10b are not particularly limited. The principal surface 10 b of thesilicon carbide single crystal substrate 10 before formation of theidentification mark 14 may have a surface roughness which is obtainedafter being finished by mechanical polishing or may be a mirror surface.

On the other hand, the principal surface 10 a preferably has a surfaceroughness which is obtained after being finished by mechanicalpolishing. This is because, when the surface roughness of the principalsurface 10 a is generally equal to a surface roughness which is obtainedafter being finished by mechanical polishing, the laser beam forformation of the identification mark 14 is prevented from passingthrough the silicon carbide single crystal substrate 10 as compared withthe case where the principal surface 10 a is a mirror surface, so thatenergy can be efficiently supplied to the principal surface 10 a of thesilicon carbide single crystal substrate 10, and the groove of theidentification mark 14 can be formed. Further, the principal surface 10a of the silicon carbide single crystal substrate 10 may be directlyirradiated with a laser beam such that the energy of the laser beam canbe supplied to the principal surface 10 a, without providing an energyabsorbing layer on the principal surface 10 a of the silicon carbidesingle crystal substrate 10 for absorbing the energy of the laser beam.Further, when the principal surface 10 a has a mirror surface beforeformation of the identification mark 14, the principal surface 10 a thatis a mirror surface is mechanically polished after formation of theidentification mark 14 as will be described later, and therefore, theprevious mirror finishing step is of no use. In view of suchcircumstances, specifically, the surface roughness Ra of the principalsurface 10 a is preferably not less than 50 nm and not more than 1000nm. To sufficiently obtain the above-described effects, more preferably,the surface roughness Ra of the principal surface 10 a is not less than100 nm and not more than 500 nm.

Then, the principal surface 10 a of the provided silicon carbide singlecrystal substrate 10 is scanned with a laser beam such that theidentification mark 14 is formed in the principal surface 10 a.Formation of the identification mark is realized by forming theidentification mark 14 which is constituted of one or more grooves bythe rough laser process (step S12) and finishing the inside of thegrooves by the finishing laser process (step S13). First, the roughlaser process (step S12) is described.

As the laser light source which emits a laser beam for formation of theidentification mark 14, a various types of laser light sources for usein laser marking may be used. Here, the laser light source may includenot only a light-emitting light source which emits laser light but alsoan optical system which is used for adapting the beam diameter, a Qswitch which is used for pulse driving of a laser beam, and a wavelengthconverter element which is used for adapting the wavelength of a laserbeam. The laser light source which is used in the present embodiment isconfigured to emit a laser beam at a wavelength which is suitable tomelting and evaporation of the silicon carbide single crystal.Specifically, the laser light source preferably emits a laser beam at awavelength of not less than 532 nm and not more than 1064 nm. A laserlight source which is configured to emit a laser beam at a wavelengthshorter than 532 nm includes an expensive oscillator and is a large-sizedevice. Therefore, particularly, the cost of forming an identificationmark is likely to increase.

The beam diameter of the laser beam emitted from the laser light sourcedepends on the size of the identification mark 14 which is to be formedand the power of the laser light source. For example, the laser lightsource emits a laser beam which has a beam diameter of, for example, notless than 5 μm and not more than 50 μm. The power of the laser lightsource is, for example, not less than 1.0 W and not more than 2.0 W.Using a laser light source whose power exceeds the upper limit is notpreferred because there is a probability that damage, such as a slip, iscaused in crystal.

The principal surface 10 a of the silicon carbide single crystalsubstrate 10 is scanned with a laser beam using a laser light sourcesuch that the identification mark 14 is formed in the principal surface10 a. The principal surface 10 a is scanned at the first energy densitysuch that a groove is formed in the principal surface 10 a of thesilicon carbide single crystal substrate 10. FIG. 3A is a plan viewschematically illustrating scanning with a laser beam. A pulsed laserbeam is emitted from the laser light source to irradiate the principalsurface 10 a with every single pulse of the laser beam which isrepresented by a beam spot 22 with the beam diameter R1 as shown in FIG.3A. To irradiate the principal surface 10 a of the silicon carbidesingle crystal substrate 10 with the laser beam at a high energydensity, the principal surface 10 a is preferably irradiated with thelaser beam such that beam spots 22 each of which is formed by a singlepulse successively overlap. As the overlapping area of the beam spots 22increases, heat can be applied to the principal surface 10 a at a higherenergy density. In this process, a groove 16 extending in a directionindicated by arrow 16 e is formed in the principal surface 10 a of thesilicon carbide single crystal substrate 10.

In the case of the silicon carbide single crystal substrate 10, to forman identification mark 14 such that it is readily perceivable by a nakedeye, the width of the groove 16 which constitutes the identificationmark 14 is preferably not less than 50 μm, and the depth of the groove16 is preferably not less than 20 μm. Usually, the beam diameter of thelaser beam is about several micrometers, which is smaller than thepreferred groove width. Therefore, it is preferred to form a groovewhich has a wider groove than the beam diameter of the laser beam bymoving the laser beam for scanning in the extending direction of thegroove 16 while the laser beam is also moved for scanning in a directionwhich is not parallel to the extending direction of the groove 16.Specifically, it is preferred to perform scanning according to ascanning pattern 24 which is zigzagged with respect to the extendingdirection of the groove 16. When scanning is performed with the laserbeam with the kerf width W2, the groove 16 is formed which has the widthW1 and which extends in a direction indicated by arrow 16 e.

By the laser beam irradiation, the silicon carbide single crystal ismelted to a predetermined depth from the principal surface 10 a of thesilicon carbide single crystal substrate 10, and the melted siliconcarbide single crystal partially evaporates. Part of the melted siliconcarbide which has not been evaporated then solidifies. As a result, thegroove 16 which constitutes the identification mark 14 is formed in theprincipal surface 10 a of the silicon carbide single crystal substrate10.

FIG. 3B shows a cross section of a groove 16 which is perpendicular tothe extending direction of the groove which is indicated by arrow 16 e.The groove 16 includes a base surface 16 c and a pair of lateralsurfaces 16 d between which the base surface 16 c extends. The basesurface 16 c and the lateral surfaces 16 d form the internal surface 16a of the groove 16. As shown in FIG. 3B, the internal surface 16 a ofthe groove 16 formed in the principal surface of the silicon carbidesingle crystal substrate 10 is formed by solidification of the meltedsilicon carbide single crystal. Also, minute solidified substances 18 ofthe solidified silicon carbide are attached onto the internal surface 16a. On the principal surface 10 a extending outside the groove 16, thereare also solidified substances or a bump 19 formed by solidification.

The solidified substances 18 produced inside the groove 16 and thesolidified substances or bump 19 produced outside the groove 16 separatefrom the silicon carbide single crystal substrate 10 and attach to theprincipal surface 10 a and the principal surface 10 b of the siliconcarbide single crystal substrate 10 as contaminants so that they cancause adverse effects, become the cause of scratches in the principalsurface 10 a or the principal surface 10 b, or turn to dust to becomethe cause of contamination of other substrates or contamination insidethe semiconductor device, in a subsequent step for fabrication of thesilicon carbide single crystal substrate 10 or a manufacture step formanufacturing a semiconductor device using the completed silicon carbidesingle crystal substrate 10. For example, when an abrasive agent whichis for use in the subsequent step that is for fabrication of the siliconcarbide single crystal substrate 10 is brought into the groove 16, theabrasive agent is trapped by the internal surface 16 a because thesurface roughness of the internal surface 16 a of the groove 16 islarge, so that there is a probability that the abrasive agent cannot beremoved from the groove 16 even by washing.

In the method for forming an identification mark on the silicon carbidesingle crystal substrate 10 according to the present embodiment, thefinishing laser process (step S13) is performed, after the rough laserprocess, on the groove 16 which has been formed by the rough laserprocess in order to solve the above problems. By the finishing laserprocess, the solidified substances 18 and the bump 19 that havepreviously been described are again melted and evaporated such that thesolidified substances 18 and the bump 19 are removed. Further, theinternal surface 16 a is melted and solidified so as to have a smoothinternal surface. The solidified substances 18, the bump 19, and theinternal surface 16 a are formed by solidification of melted siliconcarbide single crystal, so that they are amorphous or have lowcrystallinity. Further, in the rough laser process, carbon or siliconselectively evaporates, so that solidified substances 18, the bump 19,and the internal surface 16 a have a composition in which silicon orcarbon is excessively contained or a composition in which silicon orcarbon is bound to oxygen. These can be melted and evaporated even whenthe applied energy is not as large as the first energy density in therough laser process.

Thus, by scanning the inside of the groove 16 which has been formed inthe principal surface 10 a of the silicon carbide single crystalsubstrate 10 at the second energy density that is lower than the firstenergy density, the solidified substances 18 and the bump 19 can beremoved, and also, the internal surface 16 a can be smoothed. Further,since the laser beam irradiation is performed at an energy density whichis lower than the first energy density, a portion extending outside thegroove 16 which is made of silicon carbide single crystal would not benewly melted or evaporated. That is, the laser beam irradiation ispreferably performed at the second energy density such that the siliconcarbide single crystal is not melted or evaporated. Thus, production ofnew solidified substances 18 or bump 19 by the finishing laser processis prevented. It is preferred that the ratio of the total energy of thefinishing laser process to the total energy of the rough laser processis about not less than 10% and not more than 40%.

A pulsed laser beam is emitted from the laser light source to irradiatethe inside of the groove 16 formed in the principal surface 10 a withevery single pulse of the laser beam which is represented by a beam spot22′ with the beam diameter R1 as shown in FIG. 4A. In FIG. 4A, thepositions of the beam spot 22 in the rough laser process are shown bybroken lines. As seen from FIG. 4A, the overlapping area of the beamspots 22′ is smaller than that of the beam spots 22 so that the secondenergy density is smaller than the first energy density. The othermethods for decreasing the second energy density include decreasing thelaser power and increasing the scanning speed.

Preferably as shown in FIG. 4A, the scanning direction of the laser beamin the step of the rough laser process and the scanning direction of thelaser beam in the finishing laser process are different from each other.With this arrangement, the unevenness in the internal surface 16 a ofthe groove 16 which depends on the scanning direction of the laser beamin the step of the rough laser process is flattened in the finishinglaser process, so that the inside of the groove 16 can be furthersmoothed. In the present embodiment, the inside of the groove 16 isscanned with the laser beam according to a scanning pattern 24′ which isparallel to the extending direction of the groove 16. As shown in FIG.4A, it is preferred that a portion extending outside the groove 16 isalso irradiated with the beam spots 22′ in order to remove the bump 19which has been produced outside the groove 16 of the principal surface10 a. With this arrangement, the wall surface of the groove 16 is alsoirradiated with the laser beam at a sufficient energy density, wherebythe solidified substances 18 attached to the plane surface are removed,and the wall surface is smoothed. In other words, it is preferred that aregion which is irradiated with the laser beam by the finishing laserprocess entirely includes a region which is irradiated with the laserbeam in the step of the rough laser process and is also wider than aregion which is irradiated with the laser beam in the rough laserprocess. Where a region which is irradiated with the laser beam in therough laser process and a region which is irradiated with the laser beamin the finishing laser process are respectively referred to as the firstregion and the second region, the area of the second region ispreferably 100% or more of the area of the first region. To remove thebump 19 which is produced outside the groove 16, the area of the secondregion is preferably 110% or more of the area of the first region.

As shown in FIG. 4B, by the finishing laser process, the solidifiedsubstances 18 which have been formed inside the groove 16 are removed,and the internal surface 16 b of the groove 16 is further smoothed. Thebump 19 which has been produced outside the groove 16 of the principalsurface 10 a is also removed. As a result of the finishing laserprocess, the surface roughness Ra is not more than 1 μm at the internalsurface 16 b of the groove 16. Therefore, even when mechanicalpolishing, CMP (chemical mechanical polishing), or the like, is furtherperformed on the principal surface 10 a in a subsequent step, thesolidified substances 18 are prevented from separating from the grooveand causing scratches in polishing of the principal surface 10 a. Also,since the internal surface 16 b of the groove 16 is smooth, the abrasiveagent would not enter or reside in the groove 16.

The finishing laser process may be performed after the entirety of theidentification mark 14 is formed by the rough laser process and therough laser process is completed, or may be performed on parts of theidentification mark 14 which have undergone the rough laser process oneafter another. In this case, the rough laser process and the finishinglaser process are concurrently performed, and it is therefore preferredto provide a laser light source for the rough laser process and anotherlaser light source for the finishing laser process. Where the pulseinterval of the laser beam in the rough laser process is t and theinterval between the rough laser process and the finishing laser processwhich are performed in an identical region of the identification mark 14is T, T is sufficiently longer than t. That is, T≧≧t, so that part ofthe silicon carbide single crystal which is melted by the rough laserprocess is solidified and sufficiently cooled before the finishing laserprocess.

Formation of the identification mark 14 in the silicon carbide singlecrystal substrate 10 may be completed through the above-describedprocesses. Alternatively, finishing of the identification mark 14 orfinishing of the principal surface 10 a in which the identification mark14 is provided may be performed. When mechanical polishing is performedon the principal surface 10 a of the silicon carbide single crystalsubstrate 10 in which the identification mark 14 has been formed suchthat the surface roughness of the principal surface 10 a is reduced,when there is a minute bump 19′ remaining outside the groove 16 of theprincipal surface 10 a and the bump 19′ is to be removed, or when thesurface roughness of the internal surface 16 b of the groove 16 isfurther reduced, mechanical polishing is performed on the principalsurface 10 a of the silicon carbide single crystal substrate 10 (stepS14). Specifically, the principal surface 10 a of the silicon carbidesingle crystal substrate 10 is mechanically polished using a metalsurface plate and an abrasive agent. In this process, the abrasive agententers the inside of the groove 16 so that the internal surface 16 b ofthe groove 16 is also polished with the abrasive agent. As a result, asshown in FIGS. 5A and 5B, the silicon carbide single crystal substrate10 is obtained in which the surface roughness of the internal surface 16b′ is small and which has a principal surface 10 a′ with a reducedsurface roughness. Further, the bump 19′ remaining outside the groove 16also can be removed by this process.

To remove a damage layer which is formed over the surface of theprincipal surface 10 a of the silicon carbide single crystal substrate10 due to the laser beam irradiation or solidification of melted siliconcarbide, gas phase etching of the principal surface 10 a may beperformed (step S15). Examples of the etching by the gas phase methodwhich can be employed in the present embodiment include ion etching,sputter etching, reactive ion etching, plasma etching, reactive ion beametching, and ion beam etching. Any other gas phase etching method may beemployed.

The type of the gas used in the gas phase etching is not particularlylimited. However, it is preferred to use a gas which contains fluorine,such as carbon tetrafluoride or sulfur hexafluoride or a gas which hasreactivity with silicon carbide, such as hydrogen. Further, oxygen maybe added in order to enhance oxidation. The conditions for the etching,such as the power to apply, depend on the apparatus used for theetching, for example. The etching rate preferably does not exceed 10μm/h. If the etching rate exceeds 10 μm/h, the etching conditions wouldbe excessively intense for the principal surface 10 a of the siliconcarbide single crystal substrate 10 so that, probably, the principalsurface 10 a is damaged by ion collision or the surface morphology ofthe principal surface 10 a after the etching deteriorates. Thus, thisexcessive etching rate is not preferred.

Since the thickness of the damage layer is small, the etching that isbased on the gas phase method does not need to be performed for a longperiod of time. In the gas phase etching, the etching progressesgenerally uniformly so that the surface roughness of the principalsurface 10 a scarcely varies. Thus, a principal surface can be obtainedin which the surface roughness of the principal surface 10 a before thegas phase etching is generally maintained and from which the damagelayer has been removed.

If the principal surface 10 b of the silicon carbide single crystalsubstrate 10 is a surface which is finished by mechanical polishing information of the above-described identification mark, mechanicalpolishing and mirror polishing may be performed on the principal surface10 b after the formation of the identification mark.

By the method for forming an identification mark on a refractorymaterial single crystal substrate according to the present embodiment, asilicon carbide single crystal substrate 10 with an identification mark14 is obtained that is constituted of a groove 16 which has such a depthand a width that excellent visibility is achieved. Solidified substances18 and the like are scarcely remaining in the groove 16 whichconstitutes the identification mark 14, and the internal surface 16 ahas a surface roughness of not more than 1 μm. Therefore, even whenmechanical polishing, CMP (chemical mechanical polishing), or the like,is further performed on the principal surface 10 a in a subsequent step,the solidified substances 18 are prevented from separating from thegroove 16 and causing scratches in polishing of the principal surface 10a. Also, since the internal surface 16 b of the groove 16 is smooth, theabrasive agent would not enter or reside in the groove 16. Thus, anexcellent silicon carbide single crystal substrate 10 is obtained inwhich occurrence of various problems which are attributed to formationof the identification mark 14 is prevented in the process of fabricatingthe silicon carbide single crystal substrate 10 or in the process ofmanufacturing a semiconductor device.

Second Embodiment

Hereinafter, the second embodiment of a method for forming anidentification mark on a refractory material single crystal substrateaccording to the present invention is described with reference to thedrawings. In the present embodiment, a sapphire single crystal substrateis used as the refractory material single crystal substrate.

FIG. 1 schematically shows a sapphire single crystal substrate 10′ onwhich an identification mark 14 is formed by a method for forming anidentification mark on a sapphire single crystal substrate according tothe present embodiment. The sapphire single crystal substrate 10′ ismade of sapphire single crystal.

The crystal orientation of the principal surface, and the like, of thesapphire single crystal substrate 10′ are not particularly limited as isthe case with the first embodiment. The sapphire single crystalsubstrate 10′ may be a just substrate or may be an off substrate. Asdescribed in the first embodiment, the principal surface 10 b of thesapphire single crystal substrate 10′ is preferably a mirror surface. Onthe other hand, the principal surface 10 a in which an identificationmark 14 is to be formed has a surface roughness which is selectedaccording to its use or specifications required of the sapphire singlecrystal substrate 10′.

The identification mark formation method of the present embodiment forforming an identification mark on a sapphire single crystal substrate isthe same as the identification mark formation method of the firstembodiment for forming an identification mark on a silicon carbidesingle crystal substrate except that the laser irradiation conditionsare different because of the different substrate materials.Specifically, a sapphire single crystal substrate 10′ is provided aspreviously described in the first embodiment with reference to FIG. 2(step S11). The principal surface 10 a of the sapphire single crystalsubstrate 10′ is scanned with a laser beam to form an identificationmark 14 in the principal surface 10 a. Formation of the identificationmark is realized by forming the identification mark 14 which isconstituted of one or more grooves by the rough laser process (step S12)and finishing the inside of the grooves by the finishing laser process(step S13). In the case of the sapphire single crystal substrate 10′, apreferred groove depth of an identification mark with excellentvisibility is not less than 10 μm. A preferred groove width is 50 μm,which is the same as in the first embodiment. Thereafter, whennecessary, mechanical polishing is performed on the principal surface 10a of the sapphire single crystal substrate 10′ (step S14). Specifically,the principal surface 10 a of the sapphire single crystal substrate 10′is mechanically polished using a metal surface plate and an abrasiveagent. Also, when necessary, to remove a damage layer which is formedover the surface of the principal surface 10 a of the sapphire singlecrystal substrate 10′ due to the laser beam irradiation orsolidification of melted sapphire, gas phase etching of the principalsurface 10 a may be performed (step S15). As a result, the sapphiresingle crystal substrate 10′ on which an identification mark has beenformed is obtained.

Now, the different aspects of the present embodiment from the firstembodiment are mainly described. As described above, in the presentembodiment, different laser irradiation conditions are employed fromthose of the first embodiment because of the different substratematerial used. Specifically, in the present embodiment, it is preferredthat the laser beam used for formation of the identification mark 14 hasa shorter wavelength. Specifically, it is preferred that the wavelengthof the laser beam is not less than 266 nm and not more than 532 nm.

It is preferred that the ratio of the total energy of the finishinglaser process to the total energy of the rough laser process is aboutnot less than 40% and not more than 95%. Since the sapphire singlecrystal itself is an oxide as is not the case with silicon carbide, thecompositions of the solidified substances 18 and bump 19 generated bythe rough laser process and the composition of the internal surface 16 a(FIG. 3B) are not largely different from that of the aluminum oxide thatforms the sapphire single crystal substrate 10′. Therefore, the energyrequired for melting and evaporating the solidified substances 18 andbump 19 and the internal surface 16 a will not be much smaller than theenergy of the laser beam in the rough laser process. Note that, however,the solidified substances 18 and bump 19 and the internal surface 16 adeviate from the composition of Al₂O₃ to have a composition of an oxideor to be in an amorphous or polycrystalline state. Therefore, the energyrequired for melting and evaporating the solidified substances 18 andbump 19 and the internal surface 16 a may be smaller than the energyrequired for formation of a groove in the rough laser process.Adjustment of the energy density in the rough laser process and thefinishing laser process is carried out in the same way as thatpreviously described in the first embodiment.

As seen from Table 1, the thermal conductivity of sapphire is smallerthan the thermal conductivity of silicon carbide by about one order ofmagnitude. Therefore, in the case where a groove 16 is formed in thesapphire single crystal substrate 10′ by the rough laser process, heatis less likely to be transmitted to a region outside the groove which isnot irradiated with the laser beam, and accordingly, melting andevaporation of sapphire abruptly occurs inside the groove 16, whileincrease in temperature of sapphire outside the groove 16 is prevented.As a result, the state of sapphire is largely different between theinside and outside of the groove 16, and a large stress occurs at theperiphery of the groove 16 of the sapphire single crystal substrate 10′,so that cracks are likely to be generated.

To prevent generation of cracks, if the sum of the total energiessupplied in the total process of the rough laser process and finishinglaser process is constant, it is preferred that the difference in totalenergy between the rough laser process and the finishing laser processis smaller. Specifically, it is preferred that the ratio of the totalenergy for the finishing laser process to the total energy for the roughlaser process is approximately not less than 80% and not more than 95%.

By the identification mark formation method of the present embodimentfor forming an identification mark on a sapphire single crystalsubstrate, a sapphire single crystal substrate 10′ with anidentification mark 14 is obtained that is constituted of a groove 16which has such a depth and a width that excellent visibility isachieved. Solidified substances 18 and the like are scarcely remainingin the groove 16 which constitutes the identification mark 14, and theinternal surface 16 a has a surface roughness of not more than 1 μm.Therefore, even when mechanical polishing, CMP (chemical mechanicalpolishing), or the like, is further performed on the principal surface10 a in a subsequent step, the solidified substances 18 are preventedfrom separating from the groove 16 and causing scratches in polishing ofthe principal surface 10 a. Also, since the internal surface 16 b of thegroove 16 is smooth, the abrasive agent would not enter or reside in thegroove 16. Thus, an excellent sapphire single crystal substrate 10′ isobtained in which occurrence of various problems which are attributed toformation of the identification mark 14 is prevented in the process offabricating the sapphire single crystal substrate 10′ or in the processof manufacturing a semiconductor device.

Third Embodiment

Hereinafter, the third embodiment of a method for forming anidentification mark on a refractory material single crystal substrateaccording to the present invention is described with reference to thedrawings. In the present embodiment, a gallium nitride single crystalsubstrate is used as the refractory material single crystal substrate.

FIG. 1 schematically shows a gallium nitride single crystal substrate10″ on which an identification mark 14 is formed by a method for formingan identification mark on a gallium nitride single crystal substrateaccording to the present embodiment. The gallium nitride single crystalsubstrate 10″ is made of gallium nitride single crystal.

The polytype, the crystal orientation of the principal surface, and thelike, of the gallium nitride single crystal substrate 10″ are notparticularly limited as is the case with the first embodiment. Thegallium nitride single crystal substrate 10″ may be a just substrate ormay be an off substrate. As described in the first embodiment, theprincipal surface 10 b of the gallium nitride single crystal substrate10″ is preferably a mirror surface. On the other hand, the principalsurface 10 a in which an identification mark 14 is to be formed has asurface roughness which is selected according to its use orspecifications required of the gallium nitride single crystal substrate10″.

The identification mark formation method of the present embodiment forforming an identification mark on a gallium nitride single crystalsubstrate is the same as the identification mark formation method of thefirst embodiment for forming an identification mark on a silicon carbidesingle crystal substrate except that the substrate material isdifferent. Specifically, a gallium nitride single crystal substrate 10″is provided as previously described in the first embodiment withreference to FIG. 2 (step S11). The principal surface 10 a of thegallium nitride single crystal substrate 10″ is scanned with a laserbeam to form an identification mark 14 in the principal surface 10 a.Formation of the identification mark is realized by forming theidentification mark 14 which is constituted of one or more grooves bythe rough laser process (step S12) and finishing the inside of thegrooves by the finishing laser process (step S13). Thereafter, whennecessary, mechanical polishing is performed on the principal surface 10a of the gallium nitride single crystal substrate 10″ (step S14).Specifically, the principal surface 10 a of the gallium nitride singlecrystal substrate 10″ is mechanically polished using a metal surfaceplate and an abrasive agent. Also, when necessary, to remove a damagelayer which is formed over the surface of the principal surface 10 a ofthe gallium nitride single crystal substrate 10″ due to the laser beamirradiation or solidification of melted gallium nitride, gas phaseetching of the principal surface 10 a may be performed (step S15).

By the identification mark formation method of the present embodimentfor forming an identification mark on a gallium nitride single crystalsubstrate, a gallium nitride single crystal substrate 10″ with anidentification mark 14 is obtained that is constituted of a groove 16which has such a depth and a width that excellent visibility isachieved.

Other Embodiments

As seen from Table 1, not only single crystal substrates of siliconcarbide, sapphire, and gallium nitride but also single crystalsubstrates of aluminum nitride, diamond, boron nitride, zinc oxide,gallium oxide, and titanium dioxide have high melting points. Byemploying the identification mark formation methods described in thefirst to third embodiments for the single crystal substrates of thesematerials, an excellent refractory material single crystal substrate isobtained which has high visibility and in which occurrence of variousproblems that are attributed to formation of an identification mark 14is prevented.

Example 1

Hereinafter, an example of formation of an identification mark on asilicon carbide single crystal substrate with the use of a method forforming an identification mark on a silicon carbide single crystalsubstrate according to the first embodiment is described.

A 4H silicon carbide single crystal substrate with a diameter of 3inches was provided. The surface roughness Ra of the principal surfacein which an identification mark was to be formed was 0.3 μm. The laserlight source used was a Nd:YAG laser (wavelength: 1064 nm, power: 1.5 W)manufactured by ESI, Inc. This laser light source had a Q-switch and wasused to perform the rough laser process and the finishing laser processunder the conditions of Examples 1-1, 1-2, and 1-3 as shown in Table 2such that an identification mark of nine characters was formed. InComparative Example, only the rough laser process was performed forformation of an identification mark. In Table 2, the kerf width refersto W2 of FIG. 3. When the kerf width was 0, the principal surface of thesubstrate was scanned with a laser beam along a groove to be formed. Forexample, when the width of the groove was about three times the diameterof the laser beam, the principal surface was scanned with the laser beamsuch that spots in the left column shown in FIG. 3 were drawn, theprincipal surface was then scanned with the laser beam such that spotsin the center column were drawn, and lastly, the principal surface wasscanned with the laser beam such that spots in the right column weredrawn. In Table 2, the energy density is a relative value which wasdetermined with respect to the energy density of Comparative Examplewhich was assumed as 100. In Examples 1-1, 1-2, and 1-3, the energydensity of the finishing laser process was made smaller than the energydensity of the rough laser process by nulling the kerf so as to shortenthe scanning time per unit area. The total energy refers to a totalenergy which was supplied to the substrate by laser beam irradiation formarking a straight line of 1 mm in each of the rough laser process andthe finishing laser process. The energy ratio refers to the ratio of thetotal energy of the finishing laser process to the total energy of therough laser process.

After the formation of the identification mark by the laser, theprincipal surface in which the identification mark was formed wassubjected to mechanical polishing with the use of a diamond slurry inwhich diamond particles with the average particle diameter of 5 μm werecontained as the abrasive agent.

After the mechanical polishing, the inside of the groove constitutingthe identification mark was observed with an optical microscope to checkwhether there was an attached substance, such as a solidified substance,on the bottom surface and the lateral surfaces of the groove. Further,the depth and the width of the groove were measured using an opticallength-measuring microscope. The measurement was performed at anarbitrary position in the groove, where the groove width on thesubstrate surface and the groove depth from the substrate surface weremeasured. The measurement was carried out at one arbitrary position ineach of the grooves of three out of nine characters. Further, thesurface roughness Ra of the bottom surface of the groove was measuredusing the optical interference type surface roughness measuringapparatus HD-2000 manufactured by Veeco Instruments Inc. The measurementwas performed on a central portion at an arbitrary position in thegroove, along the line direction (the longitudinal direction of thegroove), in the length of about 0.2 mm. The measurement was carried outalong one arbitrary line in each of the grooves of three out of ninecharacters. The results are shown in Table 3.

TABLE 2 Comp- Exam- arative Example ple Example Example 1-1 1-2 1-3 1-1Rough Q rate (Hz) 7000 500 500 3000 Laser Power (%) 100 100 100 100Process Speed (mm/s) 40 10 15 8 Kerf Width (mm) 0 0.08 0.08 0.15 EnergyDensity 70 75 72 100 Number of 3 1 1 1 Scanning Cycles Total Energy (W)788 1200 1200 4200 Finish Q rate (Hz) 500 500 500 None Laser Power (%)100 100 100 Process Speed (mm/s) 10 10 10 Kerf Width (mm) 0 0 0 EnergyDensity 10 10 10 Number of 3 3 5 Scanning Cycles (with offset) TotalEnergy (W) 225 225 375 Energy Ratio (%) 29 19 31

TABLE 3 Example Example Example Comparative Evaluated Items 1-1 1-2 1-3Example 1-1 Attached Groove No No No Yes Substance Bottom Inside Wall NoYes No Yes Groove Surface Groove Before 45 50 50  60 Depth Polish (μm)Groove Before 45 100  110  150-170 Width Polish (μm) After 30 80 80 170Polish Surface Roughness of 0.4-0.6 0.4-0.6 0.4-0.6 5.0-7.0 GrooveBottom Surface Ra (μm)

As seen from Table 3, no attached substance was found at the bottomsurface of the groove in either of Examples 1-1, 1-2, and 1-3. InExamples 1-1 and 1-3, no attached substance was also found at the wallsurface of the groove. On the other hand, in Comparative Example,attached substances were found at the bottom surface and the wallsurface of the groove. This is probably because, in the methods ofExamples 1-1, 1-2, and 1-3, solidified substances in the groove wereremoved by the finishing laser process. It was found from the results ofExamples 1-1, 1-2, and 1-3 that attached substances can be entirelyremoved so long as the energy of the finishing laser process isapproximately not less than 19% and not more than 31% of that of therough laser process. It is understood that, when a margin of about 10%is considered, the energy ratio only needs to be approximately not lessthan 10% and not more than 40%.

In Example 1-2, the reason why there was an attached substance on thewall surface is probably that the laser beam of the finishing laserprocess failed to irradiate the wall surface inside the groove with asufficient intensity. On the other hand, in Example 1-3, the finishinglaser process was performed through five cycles, and in each scanningcycle, the position of the beam was offset by 0.02 mm. Therefore, it isinferred that the beam of the finishing laser process successfullyuniformly irradiated the entire surface inside the groove.

In each of Examples 1-1, 1-2, and 1-3, the surface roughness Ra of thebottom surface of the groove of the formed identification mark was inthe range of 0.4 μm to 0.6 μm. On the other hand, in ComparativeExample, the surface roughness Ra of the bottom surface of the groovewas in the range of 5.0 μm to 7.0 μm. It was found from this result thatthe surface roughness of the internal surface of the groove of theidentification mark which was formed according to the methods ofExamples 1-1, 1-2, and 1-3 was improved to about 1/10 of the surfaceroughness of the groove which was formed according to the conventionalmethod. In Examples 1-1, 1-2, and 1-3 and Comparative Example, reductionof the surface roughness Ra by the mechanical polishing is estimated atabout 50 nm to 100 nm, and therefore, the above-described difference insurface roughness Ra is not attributed to the mechanical polishing whichis performed after the formation of the identification mark. In Examples1-1, 1-2, and 1-3, it can be said that the surface roughness Ra of thebottom surface of the internal surface of the groove of theidentification mark before the mechanical polishing is at least not morethan 1 μm.

It was confirmed that the identification marks of Examples 1-1, 1-2, and1-3 had improved visibility for a naked eye as compared with ComparativeExample. It was also confirmed that the identification marks which wereformed by the methods of Examples 1-2 and 1-3 had further improvedvisibility for a naked eye as compared with the identification markwhich was formed by the method of Example 1-1.

FIG. 6 is an enlarged SEM image showing a portion of a groove of anidentification mark which was formed by the method of Example 1-3. Asseen from FIG. 6, there was substantially no contaminant on the bottomsurface and the lateral surfaces of the groove. Also, there wassubstantially no unevenness in the internal surface of the groove, andit is appreciated that the surface roughness of the internal surface wasvery small. Particularly, it can be seen that the bottom surface of thegroove was a solidified surface, and it had a striped pattern which wasgenerally perpendicular to the extending direction of the groove. Thisis probably because a solidified substance was removed by the finishinglaser process, and as a result, traces were produced at the bottomsurface of the groove due to sequential melting and solidification ofsilicon carbide single crystal along the traveling direction of the beamspot in the rough laser process, i.e., the scanning direction of thelaser beam. Formation of a solidified surface over the internal surfaceof the groove prevented minute contaminants from remaining on thesurface.

It is also seen that edges which defined the groove were also sharp, andthe principal surface extending outside the groove was flat. It wasconfirmed that an identification mark having a desired shape was alsoformed by the method of Example 1-1. In view of these circumstances, weconsider that the identification mark formation methods of Examples 1-1and 1-3 are more preferred among Examples 1-1, 1-2, and 1-3.

FIGS. 7A and 7B respectively show surface profiles of a cross sectionperpendicular to, and a cross section parallel to, the extendingdirection of a groove of an identification mark which was formed by themethod of Example 1-3. As seen from these graphs, of the internalsurface of the groove, at least the bottom surface had a surfaceroughness Ra of not more than 1 μm.

FIG. 8 is an enlarged SEM image showing a portion of a groove of anidentification mark which was formed by the method of ComparativeExample 1-1. As seen from FIG. 8, there were a large number of smallsolidified substances attached onto the internal surface of the grooveso that the internal surface of the groove had an uneven shape. Also,the principal surface extending outside the groove was not flat but hadbumps. FIGS. 9A and 9B respectively show a surface profile along adirection perpendicular to the extending direction of a groove of anidentification mark formed by the method of Comparative Example 1-1 anda surface profile along the extending direction of that groove. As seenfrom these graphs, the internal surface of the groove had a surfaceroughness Ra of not less than several tens of micrometers, so that theinternal surface of the groove was not smooth.

From the above results, it was found that an identification markconstituted of a groove which has no contaminant attached onto theinternal surface and of which the internal surface is very smooth can beformed by the method of Example 1 in which the rough laser process andthe finishing laser process are performed. Employing a mark which is inthe form of a groove rather than dots contributes to excellentvisibility. It was found that, from the viewpoint of visibility, a kerfwidth is provided, and the scanning pattern of the laser beam iszigzagged in such a manner that a groove width of not less than about 50μm is secured, whereby an identification mark with excellent visibilitycan be formed.

Example 2

Hereinafter, an example of formation of an identification mark on asapphire single crystal substrate with the use of a method for formingan identification mark on a refractory material single crystal substrateaccording to the second embodiment is described.

A sapphire single crystal substrate with a diameter of 4 inches wasprovided. The laser light source used for Example 2-1 and ComparativeExample 2-1 was a laser manufactured by MIYACHI CORPORATION (wavelength:532 nm, power: 5 W). The laser light source used for Example 2-2 was alaser manufactured by OMRON Corporation (wavelength: 355 nm, power: 0.4W). The laser light source used for Example 2-3 was a laser manufacturedby Takano Co., Ltd. (wavelength: 266 nm, power: 0.25 W). The laser lightsource used for Comparative Example 2-2 was a Nd:YAG laser (wavelength:1064 nm, power: 1.5 W) manufactured by ESI, Inc. These laser lightsources had a Q-switch and were used to perform the rough laser processand the finishing laser process under the conditions of Examples 2-1 and2-2 as shown in Table 4 such that an identification mark of ninecharacters was formed. In Comparative Examples 2-1 and 2-2, only therough laser process was performed for formation of an identificationmark. In Table 4, the kerf width refers to W2 of FIG. 3. When the kerfwidth was 0, the principal surface of the substrate was scanned with alaser beam along the extending direction of a groove to be formed. Forexample, when the width of the groove was about three times the diameterof the laser beam, the principal surface was scanned with the laser beamsuch that spots in the left column shown in FIG. 3A were drawn, theprincipal surface was then scanned with the laser beam such that spotsin the center column were drawn, and lastly, the principal surface wasscanned with the laser beam such that spots in the right column weredrawn. In Table 4, the energy density is a relative value which wasdetermined with respect to the energy density of Comparative Example 2-2which was assumed as 100. In Examples 2-1, 2-2, and 2-3, the energydensity of the finishing laser process was made smaller than the energydensity of the rough laser process by decreasing the power. The totalenergy refers to a total energy which was supplied to the substrate bylaser beam irradiation for marking a straight line of 1 mm in each ofthe rough laser process and the finishing laser process. The energyratio refers to the ratio of the total energy of the finishing laserprocess to the total energy of the rough laser process.

After the finishing laser process, the mechanical polishing was notperformed. The inside of the groove constituting the identification markwas observed with an optical microscope to check whether there was anattached substance, such as a solidified substance, on the bottomsurface and the lateral surfaces of the groove. Further, the depth andthe width of the groove were measured using an optical length-measuringmicroscope. The measurement was performed at an arbitrary position inthe groove, where the groove width on the substrate surface and thegroove depth from the substrate surface were measured. The measurementwas carried out at one arbitrary position in each of the grooves ofthree out of nine characters. Further, the surface roughness Ra of thebottom surface of the groove was measured using the optical interferencetype surface roughness measuring apparatus HD-2000 manufactured by VeecoInstruments Inc. The measurement was performed on a central portion atan arbitrary position in the groove, along the line direction (thelongitudinal direction of the groove), in the length of about 0.2 mm.The measurement was carried out along one arbitrary line in each of thegrooves of three out of nine characters. The results are shown in Table5.

TABLE 4 Exam- Exam- Exam- Comp. Comp. ple ple ple Ex. Ex. 2-1 2-2 2-32-1 2-2 Laser Wavelength 532 355 266 532 1064 (nm) Rough Q rate (Hz)30000 1000 10000 1000 3000 Laser Power (%) 100 100 100 80 100 ProcessSpeed 100 1 1 8 8 (mm/s) Kerf 0.15 0 0 0.15 0.15 Width (mm) Energy 40 6040 40 100 Density Number 2 1 7 1 1 of Scan Cycles Total 7000 4500 70001100 4200 Energy (W) Finish Q rate (Hz) 40000 3000 10000 None None LaserPower (%) 80 80 50 Process Speed 10 3 1 (mm/s) Kerf 0 0 0 Width (mm)Energy 10 20 20 Density Number 1 1 7 of Scan Cycles Total 6000 3000 3500Energy (W) Energy 85 67 50 Ratio (%)

TABLE 5 Exam- Exam- Exam- Comp. Comp. ple ple ple Ex. Ex. EvaluatedItems 2-1 2-2 2-3 2-1 2-2 Cracks Near Groove No No No very Yes smallAttached Groove No No No Yes Yes Substance Bottom Inside Wall No No NoYes Yes Groove Surface Groove Before  10  40 10 50 Depth (μm) PolishGroove Before 150 130 90 150-170 Width (μm) Polish After Polish SurfaceRoughness 0.6-0.8 0.5-0.7    0.76 1-2 3-5 of Groove Bottom Surface Ra(μm)

As seen from Table 5, in Examples 2-1, 2-2, and 2-3, no crack was formednear the groove, and no attached substance was found at the bottomsurface or the wall surface of the groove. On the other hand, inComparative Examples 2-1 and 2-2, cracks were formed near the groove,and attached substances were found at the bottom surface and the wallsurface of the groove. This is probably because, in the methods ofExamples 2-1, 2-2, and 2-3, solidified substances in the groove wereremoved by the finishing laser process. It was found from the results ofExamples 2-1, 2-2, and 2-3 that attached substances can be entirelyremoved so long as the energy of the finishing laser process isapproximately not less than 50% and not more than 85% of that of therough laser process. It is understood that, when a margin of about 10%is considered, the energy ratio only needs to be approximately not lessthan 40% and not more than 95%.

In Examples 2-1, 2-2, and 2-3, the surface roughness Ra of the bottomsurface of the groove of the identification mark was in the range of 0.5μm to 0.8 μm. On the other hand, in Comparative Examples 2-1 and 2-2,the surface roughness Ra of the bottom surface of the groove was in therange of 1.0 μm to 5 μm. It was found from this result that the surfaceroughness of the internal surface of the groove of the identificationmark which was formed according to the methods of Examples 2-1 and 2-2was improved to, at the maximum, about 1/10 of the surface roughness ofthe groove which was formed according to the conventional method. It wasalso found that the surface roughness Ra of the bottom surface of thegroove of the identification mark at least in Examples 2-1, 2-2, and 2-3was not more than 1 μm.

It was confirmed that the identification marks of Examples 2-1, 2-2, and2-3 had improved visibility for a naked eye as compared with ComparativeExamples 2-1 and 2-2.

FIG. 10 is an enlarged optical microscopic image showing a portion of agroove of an identification mark which was formed by the method ofExample 2-1. As seen from FIG. 10, no crack was formed near the groove,and there was substantially no contaminant on the bottom surface and thelateral surfaces of the groove. It is also seen that the identificationmark can be observed with high visibility although the depth of thegroove was 10 μm. FIGS. 11A and 11B show surface profiles of a crosssection perpendicular to, and a cross section parallel to, the extendingdirection of a groove of an identification mark which was formed by themethod of Example 2-1. The surface roughness Ra of the bottom surface ofthe groove was not more than 1 μm.

FIG. 12 is an enlarged optical microscopic image showing a portion of agroove of an identification mark which was formed by the method ofExample 2-2. As seen from Table 4, in Example 2-2, the kerf width was 0.This is why scanning with the laser beam was carried out along theextending direction of the groove, and a plurality of scanning tracesextending along the groove of the identification mark were also seen inthe groove. FIGS. 13A and 13B show surface profiles of a cross sectionperpendicular to, and a cross section parallel to, the extendingdirection of a groove of an identification mark which was formed by themethod of Example 2-2. As seen from FIG. 13A, in the profile in thedirection perpendicular to the groove of the identification mark, thereare a plurality of grooves. However, as seen from FIG. 13B, the surfaceroughness Ra of the bottom surface along the extending direction of thegroove is approximately not more than 1 μm.

FIG. 14 is an enlarged optical microscopic image showing a portion of agroove of an identification mark which was formed by the method ofComparative Example 2-1. As seen from FIG. 14, there are cracks in aperipheral region of the groove. The edge of the groove has an irregularshape, so that the edge of the groove is not shaped as designed.Therefore, it is very difficult to discern it as a mark.

From the above results, it was found that an identification markconstituted of a groove which has no contaminant attached onto theinternal surface and of which the internal surface is very smooth can beformed by the method of Example 2 in which the rough laser process andthe finishing laser process are performed. Employing a mark which is inthe form of a groove rather than dots contributes to excellentvisibility.

The present invention is suitably applicable to a refractory materialsingle crystal substrate which is used in various uses, includingmanufacture of a semiconductor device.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

This application is based on Japanese Patent Application No. 2012-154487filed on Jul. 10, 2012, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A method for forming an identification mark on a refractory material single crystal substrate, the refractory material single crystal substrate being made of a single crystal which is formed by one selected from the group consisting of sapphire, gallium nitride, aluminum nitride, diamond, boron nitride, zinc oxide, gallium oxide, and titanium dioxide, the method comprising: (a) scanning a principal surface of the refractory material single crystal substrate with a laser beam at a first energy density such that a groove is formed in the principal surface of the refractory material single crystal substrate, thereby forming an identification mark which is constituted of one or more grooves in the principal surface of the refractory material single crystal substrate; and (b) scanning an inside of the groove formed in the principal surface of the refractory material single crystal substrate with a laser beam at a second energy density that is lower than the first energy density.
 2. The method of claim 1, wherein a width of the groove is not less than 50 μm, and a depth of the groove is not less than 10 μm.
 3. The method of claim 1, wherein the surface roughness Ra of an internal surface of the groove is not more than 1 μm.
 4. The method of claim 1, further comprising (c) after step (b), performing mechanical polishing on the principal surface of the refractory material single crystal substrate.
 5. The method of claim 4 wherein, after step (c), gas phase etching is performed on the principal surface of the refractory material single crystal substrate.
 6. The method of claim 1, wherein the surface roughness Ra of the principal surface of the refractory material single crystal substrate is not less than 0.1 nm and not more than 2.0 nm.
 7. A refractory material single crystal substrate which has an identification mark on a principal surface, the identification mark being constituted of one or more grooves, and the refractory material single crystal substrate being made of a single crystal which is formed by one selected from the group consisting of sapphire, gallium nitride, aluminum nitride, diamond, boron nitride, zinc oxide, gallium oxide, and titanium dioxide, wherein a width of the groove is not less than 50 μm and less than 0.5 mm, and a depth of the groove is not less than 10 μm, and a surface roughness Ra of an internal surface of the groove is not more than 1 μm.
 8. The refractory material single crystal substrate of claim 7, wherein the surface roughness Ra of the principal surface is not less than 0.1 nm and not more than 2.0 nm.
 9. The refractory material single crystal substrate of claim 7, wherein a bottom surface of the groove is a solidified surface.
 10. The refractory material single crystal substrate of claim 9, wherein the bottom surface of the groove has a striped pattern.
 11. The refractory material single crystal substrate of claim 8, wherein a bottom surface of the groove is a solidified surface.
 12. The refractory material single crystal substrate of claim 11, wherein the bottom surface of the groove has a striped pattern. 